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The Hikurangi Margin (HM) is a subduction zone along the east coast of the North Island of New Zealand where varying instances of slow slip events (SSEs) and earthquakes occur. These SSEs occur at different time scales and depths when comparing the northern and southern ends of the margin. Previous studies show that the rock comprising the accretionary wedge of the northern margin have low permeabilities, which could induce overpressures and modulate the occurrence of SSEs. Permeability rises when an SSE fractures the rocks within the deep wedge promoting fluid flow and thus dissipating the overpressures along and above the décollement. As fractures heal and permeability recovers overpressures build up once again. Although this cycle may explain the occurrence of SSEs along northern Hikurangi, it is not yet clear how intrinsic permeability varies in rocks above the décollement elsewhere along the margin. To better understand the disparity in SSE occurrence, rock samples from the northern and central part of the margin have been tested for permeability and elastic properties. We tested samples from the Weber, Whangai, Dannevirke and Wanstead formations, which are representative of the lithologies above the décollement in the central margin, and range in age from the Cretaceous to the mid to late Paleogene. We found that the Weber (PQ) and Whangai (PO) formation samples from central HM have higher permeability than northern HM rocks from the same formation in the north. This study provides insight into the mechanisms that lead to significantly fewer SSEs along the central HM. In the near future, we plan to conduct a suite of physical experiments that will include permeability recovery after fracturing, compaction, and ultrasonic velocity analysis to help further understand the stark differences in slip behavior observed along the margin. 

New Zealand's Hikurangi margin is known for recurring shallow slow slip, numerous forearc seeps, and a productive volcanic arc. Fluids derived from the subducting slab are implicated in these processes. However, prior studies lacked evidence of basic crustal structure of the slab, or of its water content that would allow an assessment of fluid budgets. We review several recent studies that place bounds on the fluid reservoirs within the subducting Hikurangi Plateau that could be released between the forearc and backarc regions. Subducting sediments are thickest (> 1 km) in the southern Hikurangi margin, where there is a unit of turbidites beneath the regional proto decollement. These subducting sediments begin draining near the deformation front, resulting in a 20-30 % loss of volumetric fluid content. In contrast, the central and northern Hikurangi margins lack a continuous unit of subducting sediment. Here, lenses of poorly drained sediment underthrust the forearc in the wakes of seamount collisions. The Hikurangi Plateau's crustal structure resembles normal oceanic crust with a doubled upper crust of basalt and diabase. Above this upper crust is a ~1.5 km thick unit of hydrated volcaniclastic conglomerates. Seamounts can locally increase the upper crust's thickness by an extra ~1-3 km, raising the amount of porous, altered volcanic material. Finally, P-wave velocity models of the slab's upper mantle show velocity changes that could indicate moderate differences in serpentinization. Active bend-faults that could circulate fluids to the upper mantle are sparse prior to subduction. However, upon subduction the upper mantle seismic velocities of the Hikurangi Plateau are significantly less in the north compared to the south, possibly due to enhanced slab faulting beneath the forearc. Separate thermo-petrologic models for the shallow forearc and deeper subduction system suggests that fluid release from volcaniclastic units and the thickened Hikurangi Plateau upper crust is expected to occur over a range of depths extending from ~12 km to ~130 km, providing fluids for onshore seep systems and hydrous melting of the mantle wedge, whereas dehydration of serpentinite is greatest beyond the arc front. Subducting sediments and volcaniclastic units are the most readily available source of fluids for shallow slow slip. 

Abstract The southern Hikurangi subduction zone exhibits significant along‐strike variation in convergence rate and obliquity, sediment thickness and, uniquely, the increasing proximity of southern Hikurangi to, and impingement on, the incoming continental Chatham Rise, an ancient Gondwana accretionary complex. There are corresponding changes in the morphology and structure of the Hikurangi accretionary prism. We combine widely spaced multichannel seismic reflection profiles with high resolution bathymetry and previous interpretations to characterize the structure and the history of the accretionary prism since 2 Ma. The southern Hikurangi margin can be divided into three segments. A northeastern segment (A) characterized by a moderately wide (∼70 km), low taper (∼5°) prism recording uninhibited outward growth in the last ∼1 Myr. Deformation resolvable in seismic reflection data accounts for ∼20 % of plate convergence, comparable with the central Hikurangi margin further North. A central segment (B) characterized by a narrow (∼30 km), moderate taper (∼8°) prism, with earlier (∼2‐∼1 Ma) shortening than segment A. Outward prism growth ceased coincidentally with development of major strike‐slip faults in the prism interior, reduced margin‐normal convergence rate, and the onset of impingement on the incoming Chatham Rise to the south. A southwestern segment (C) marks the approximate southern termination of subduction but widens to ∼50 km due to rapid outward migration of the deformation front via fault reactivation within the now‐underthrusting corner of the Chatham Rise. Segment C exhibits minimal shortening as margin‐normal subduction velocity decreases and plate motion is increasingly taken up by interior thrusts and strike‐slip faults. 

The northern part of the Hikurangi margin (HM) regularly experiences shallow slow-slip events (SSEs), possibly extending into the thrust faults of the sedimentary prism. For example, offshore Gisborne SSEs occur every 1-2 years and can last several weeks, during which 5-30 cm of slip may be accommodated. Understanding what controls the timing of such events will help the comprehension of HM deformation and earthquake mechanics in general. One hypothesis for a slow slip mechanism is that the low permeability of the HM prism rocks and the large fluid volumes dragged deep into the subduction zone cause over-pressures along the megathrust and prism splay faults. Overpressure induces SSEs that locally increase permeability. After an SSE, swelling clays and ductile deformation reduce permeability within months, resetting the conditions for developing overpressure. We tested such a hypothesis by measuring the hydraulic permeability of fractured sedimentary rocks making up the core of the accretionary prism. Tests were performed using a newly developed X-ray transparent pressure vessel mounted inside a micro-computed tomography scanner (mCT) that allowed in-situ observation of fracture evolution as a function of confining pressure, time, and exposure to water. The tested rocks were probably subducted to ~7.5 km and are calcareous-glauconitic fine-grained sandstones with a silty matrix from the Late Cretaceous-to-Paleocene Tinui Group containing ~15% vol% of clay minerals. After exposure to high confining pressure and water, the samples regained pre-fracture permeability in tens of days. mCT imagery suggests that fracture clogging, possibly due to clay expansion, controls healing. We propose that slow slip events in the northern HM open fault fractures and allow drainage at the beginning of the slip cycle, followed by fracture clogging due to swelling clays and ductile deformation, with the duration of the cycle regulated by the interplay of these processes. 

The Hikurangi margin of New Zealand exhibits contrasting slip behavior from south to north. Whereas the southern Hikurangi margin has a locked plate boundary that can potentially produce large megathrust earthquakes, the northern section of this margin accommodates plate motion by creep and recurring shallow slow-slip events. To investigate these different modes of slip we use marine seismic reflection data to image the reflectivity and seismic velocity structure along profiles across the accretionary wedge. Seismic veloc¬ity images up to 12 km deep and prestack depth migrations together charac¬terize the nature of incoming basement, sediment subduction and accretion, and faulting and compaction of the accretionary wedge. Our seismic velocity models show that a layer of sediment,with seismic wavespeeds of ~3.5 km/s, is entrained beneath the accretionary prism in the southern Hikurangi margin, but there is no coherent subducted sediment layer to the north. This is a significant result, because it implies that the sedi¬ment layer covers basement roughness and forms a smoother plate boundary in the south. In addition, the deepest sediments on the incoming plate in the southern Hikurangi margin are believed to be quartz-rich turbidites, which are prone to unstable slip along the plate boundary. In contrast, the accre¬tionary prism of the northern Hikurangi margin exhibits more variation in accretionary wedge thrust geometry due to interactions with large seamounts on the downgoing oceanic basement. These findings are consistent with the geodetically locked nature of a smooth, quartz-rich plate boundary along the southern Hikurangi subduction zone, and the creeping nature of a heteroge¬neous plate boundary along the Hikurangi margin to the north. 

Two decades of onshore-offshore, ocean bottom seismometer and marine multi-channel seismic data are integrated to constrain the crustal structure of the entire Hikurangi subduction zone. Our method provides refined 3-D constraints on the width and properties of the frontal prism, the thickness and geological architecture of the forearc crust, and the crustal structure and geometry of the subducting Hikurangi Plateau to 40 km depth. Our results reveal significant along-strike changes in the distribution of rigid crustal rocks in the overthrusting plate and along-strike changes in the crustal thickness of the subducting Hikurangi Plateau. We also provide regional constraints on seismic structure in the vicinity of the subduction interface. In this presentation, we will describe our observations and integrate our tomographic model with residual gravity anomalies, onshore geology, and geodetic observations to describe the relationship between crustal structure and fault-slip behavior along the Hikurangi margin. 

The Hikurangi margin of New Zealand exhibits contrasting slip behavior from south to north. Whereas the southern Hikurangi margin has a locked plate boundary that can potentially produce large megathrust earthquakes, the northern section of this margin accommodates plate motion by creep and episodic shallow slow-slip events. To investigate these different modes of slip we examine the geometry of the plate boundary and consolidation state of the materials along the plate interface. We use marine seismic reflection data from the SHIRE project to image the reflectivity and seismic velocity structure along 20 profiles across the accretionary wedge of the Hikurangi subduction zone of New Zealand. These active-source seismic data were gathered in 2017 with the R/V Marcus Langseth using a 6,600 in3 seismic source and 12 km long receiver array. We carried out streamer tomography on the SHIRE profiles where we integrated seismic velocity constraints from stacking the reflection data along all SHIRE transects. The seismic velocity images and prestack depth migrations together characterize the nature of incoming basement, sediment subduction and accretion, and faulting and compaction of the accretionary wedge. Our seismic velocity models show that a layer of sediment,with seismic wavespeeds of ~3.0 km/s, is entrained beneath the accretionary prism in the southern Hikurangi margin, but there is no coherent subducted sediment layer to the north. This is a significant result, because it implies that the sediment layer covers basement roughness and forms a smoother plate boundary in the south. In addition, the deepest sediments on the incoming plate in the southern Hikurangi margin are believed to be quartz-rich turbidites, which are prone to unstable slip along the plate boundary. In contrast, the accretionary prism of the northern Hikurangi margin exhibits more variation in accretionary wedge thrust geometry due to interactions with large seamounts on the downgoing oceanic basement. These findings are consistent with the geodetically locked nature of a smooth, quartz-rich plate boundary along the southern Hikurangi subduction zone, and the creeping nature of a heterogeneous plate boundary along the Hikurangi margin to the north. 

The Hikurangi subduction zone exhibits a north-to-south variation in deformation style. The plate interface in the south is locked, and megathrust earthquakes could accommodate the long-term plate convergence. In contrast, the northern megathrust regularly experiences shallow slow-slip events possibly extending into the thrust faults of the sedimentary prism. Understanding such a difference could reveal slip behavior and seismic cycle controls and help earthquake forecasting globally. One hypothesis is that the prism rock properties and fluid pressures affect these different slip behaviors. To test such a hypothesis, we measured the physical properties of rocks from the northern Hikurangi margin, focusing on ultrasonic elastic properties, permeability, and fracture healing. Such lithologies are equivalent to rocks buried to a few km depths within the accretionary prism. We found that all rocks contain >18 vol% of clay minerals. The hydraulic permeability of rock samples that are proxies for the deep part of the prism (i.e., 5 to 10 km depth) is three to four orders of magnitude lower than the values estimated by different authors for the prism as a whole. The results suggest that active faults and fractures in the accretionary prism must play a key role in draining fluids from the base of the prism and potentially from the subducting plate. Tests on a fractured sample show that fractures heal in tens of days, and permeability decreases over a short period relative to slip cycles of just a few weeks. Microphotography and micro-CT images suggest that healing is achieved by clay expansion. The observed healing could be underestimated as achieved under high confining pressure (up to 200 MPa) but at room temperature and humidity. We conclude that slow slip events in the northern Hikurangi margin may have a critical role in briefly increasing permeability at the beginning of the slip cycle, thus regulating pore pressure in the prism and allowing drainage.